An organic electroluminescence device including an organic thin-film layer (in which an emitting layer is included) between an anode and a cathode, has been known to emit light using exciton energy generated by a recombination at the organic thin-film layer of holes injected from the anode into the emitting layer and electrons injected from the cathode into the emitting layer.
Such an organic electroluminescence device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
In an optical design of such an organic electroluminescence device, an optical coherence length is adjusted in order to enhance luminous efficiency. By adjusting a thickness of organic layers such as a hole transporting layer, the luminous efficiency can be effectively enhanced and luminous spectrum can be modulated. Thus, adjustment of the optical coherence length is requisite in designing a device.
However, the light trapped within the device cannot be extracted only by adjusting the optical coherence length. Thus, a device that has an arrangement for efficiently extracting the light trapped within the device to drastically enhance the luminous efficiency has been proposed.
A loss of light in a device arrangement for extracting the light in a direction in which a light-transmissive support base (light-transmissive body) for supporting an organic thin-film layer is located is roughly classified into the following modes.
(i) a mode of light trapped in the light-transmissive body by a total reflection at an interface between the light-transmissive body and air (substrate mode)
(ii) a mode of light trapped in a transparent electrode and an organic layer by a total reflection at an interface between the transparent electrode and the light-transmissive body (thin-film mode)
(iii) a mode of light absorbed by a metal electrode as a surface plasmon (surface plasmon mode)
These lossy modes account for several tens of percents to nearly one hundred percents of a total luminous energy in the organic emitting layer depending on the conditions of luminous molecules in the organic emitting layer. Accordingly, it is necessary to develop a mechanism for extracting light of these lossy modes to an outside in order to develop an organic electroluminescence device that emits at a high efficiency.
All of the entrapments of the light according to the substrate mode, the thin-film mode and the surface plasmon mode occur because the light radiated at the emitting layer is trapped within the device as evanescent light.
Various methods for extracting the light trapped within the device as evanescent light have been reported.
In order to extract the light in the substrate mode, it is proposed to provide a convexo-concave structure such as light-scattering particles and microlenses on a surface of a light-transmissive support base. For instance, in Patent Literature 1, an electrode, an electroluminescence layer and a light-transmissive body are layered in this order and layer having a light-scattering function is provided on a light-extraction surface of the light-transmissive body. By providing light-scattering particles at an interface between the light-transmissive body and air, the light in the substrate mode is extracted. Alternatively, in an embodiment, a device arrangement in which a scattering layer and the like for changing the direction of the light to restrain the total reflection is provided within the light-transmissive body is also proposed.
In order to extract the light in the thin-film mode, a device structure in which a light-scattering layer is provided between a light-transmissive body and a transparent electrode layer is proposed. The light-scattering layer is provided by dispersing high refractive index particles of titania and the like having a particle size of several tens of nanometers to several tens of micrometers in a binder resin, by a porous silica layer or by various new light-scattering materials.
For instance, in Patent Literature 2, an electrode, an electroluminescence layer, a high refractive index transparent electrode layer and a light-transmissive body are layered in this order and a scattering layer for evanescent light is provided on respective light-extraction surfaces of the high refractive index transparent electrode layer and the light-transmissive body. The scattering layer for evanescent light is a layer containing particles for scattering the light in a matrix of a low refractive index material. The scattering layer for evanescent light has a light-scattering function for the evanescent light and extracts the light in the thin-film mode. The “light-scattering function” means a multiply scattering light rays by Mie scattering, which is specifically defined as scattering a waveguide light traveling inside an organic thin-film layer in a light-extracting direction. Incidentally, the method used in Patent Literature 2 provides poor flatness of the interface between the light-extraction layer and the transparent electrode. Accordingly, a method for providing an organic electroluminescence device with more reliability and higher light-extraction efficiency is demanded.
In Patent Literature 3, a method in which a convexo-concave structure such as a microlens is interposed between a light-transmissive electrode and a light-transmissive substrate to restrain a total reflection of light is proposed.
Further, a method for enhancing the light-extraction efficiency by providing a diffraction grating having a minute (e.g. submicron order) periodic structure or a photonic crystal to control dispersion relationship (a relationship between energy and wavenumber) of light has been vigorously studied. For instance, in Patent Literature 4 or 5, organic electroluminescence devices having a diffraction grating portion with a periodicity of approximately submicron order between light-transmissive substrate and a first electrode are proposed. The diffraction grating provided in these organic electroluminescence devices allows extraction of the light trapped as evanescent light (light in thin-film mode).